Re-discussion on the essence of the ultra-bright fluorescent carbon dots synthesized by citric acid and ethylenediamine

Citric acid and ethylenediamine can synthesize ultrabright fluorescent carbon dots (super CDs, S-CDs) with photoluminescence quantum yields (PLQY) up to ∼100% by simple hydrothermal method, almost exceeding all the currently reported fluorescent carbon dots. In this work, we examined the synthesis and properties of S-CDs in details. Our observations suggest that carbonization/graphitization degree contributes little to the PLQY of S-CDs. Morphological characterization shows the S-CDs have no graphene cores. Excitation wavelength independent emission behavior implies that the S-CDs might have an exclusive emission center/fluorophore, which is independent of the quantum size effect. Dialysis experiment reveals that not only nanoparticles, small molecular fragments also have PL emission ability. All the above experimental observations suggest that the S-CDs should be not real CDs but copolymer of CA and EDA. Furthermore, a possible reaction mechanism was proposed for synthesis of the S-CDs. This work reveals the essence of S-CDs and paves the way for their large-scale industrial production and applications.